Particle therapy, and specifically particle beam therapy using magnetic scanning or positioning of mono-energetic pencil beams is understood to offer advantages over other methods of treating localized cancer tumors. In pencil beam scanning, a narrow beam of ions such as protons or carbon nuclei, of known energy, intensity and transverse intensity distribution is directed to a notional treatment position called the isocenter. The beam spot is moved laterally across an isocenter plane to “paint” a layer or lateral distribution of therapeutic radiation dose according to a map derived from a treatment plan. This exercise is repeated at a number of beam kinetic energies, in the typical range 50 to 250 MeV for protons, so that the particles stop and deliver most of their dose at defined depths inside the patient, thus building up a volumetric distribution that is made to conform to the target tumor. In recent developments, the beam energy may be adjusted more frequently than every layer.
It is vital that the therapist can be sure that the system delivers therapeutic beams of radiation that are within acceptable tolerances of the beams assumed in the treatment plan. This requires knowledge of the position, shape, transverse intensity distribution, trajectory, divergence, delivered charge and kinetic energy of the particle beam. The same knowledge is required during initial commissioning of the system, when a large number of beam kinetic energy and delivery angles are characterized to build up a database of settings. This database can then be used to recall particular beams, and also provides input into the treatment planning system. This is a very time-consuming process that limits the rate of commissioning new particle therapy facilities. Existing beam quality measurement systems used in particle therapy are generally the same as, or derived from, prior equipment used to qualify X-ray radiotherapy equipment. While this may be a rational approach for the particle beam delivery system known as double scattering, it is inappropriate for pencil beam scanning.
X-ray radiotherapy beams are nominally uniform over the area being treated. Double-scattering particle therapy can be considered to be approximately the same in that the objective is to achieve uniformity over some area, and the beam energy is usually modulated quickly so that the deposition of dose in the patient is spread longitudinally to form the spread out Bragg peak (SOBP). The beam can therefore be considered as smoothly distributed in space and constant in time. Thus many prior art quality assurance methods involve the use of small ionization chambers immersed in a tank of water that simulates absorption in body tissues. Localized measurements can be reasonably assumed to represent the overall dose distribution. In pencil beam particle therapy this is not the case. The beam current, position and energy, and even shape can all be deliberately adjusted, or may alter. The beam quality measurement problem is local.
Prior art systems that have sought to address the particular needs of pencil beam scanning focus on only a part of the whole problem, or have shortcomings. For example, they may record the beam position but not its shape or trajectory. They may be fixed to the particle beamline so that they rotate with it, but this does not detect any errors in the rotation relative to the patient coordinate system. They may detect the beam current but not its energy. They may detect the beam shape in projection onto two orthogonal axes, but not its true two-dimensional profile. There is no prior art system that measures all the key parameters at the same time, as required for best quality assurance and speed.